In recent years, display devices have been used in various electronic devices such as television receivers, personal computer monitors, and smart phones, and the performance of the display devices has been improved to achieve higher definition, lower power consumption, and other various objectives.
In addition, semiconductor devices such as central processing units (CPUs), memories, and sensors have been used in various electronic devices such as personal computers, smart phones, and digital cameras. The performance of the semiconductor devices has also been improved to achieve miniaturization, lower power consumption, and other various objectives.
One of the ways that have been proposed to achieve higher performance such as higher definition, lower power consumption, and miniaturization is the use of an oxide semiconductor for a semiconductor layer (hereinafter also referred to as an active layer, a channel layer, or a channel formation region) of a transistor in a semiconductor device. An example of the transistor includes an oxide of indium, gallium, and zinc (hereinafter also referred to as an In—Ga—Zn oxide) for a channel layer (see Patent Document 1).
Patent Document 2 discloses a memory device including an oxide semiconductor transistor and a single crystal silicon transistor.